There are a variety of lamps used in the lighting industry. Some examples are high intensity discharge (HID), fluorescent, LED and incandescent. Each of these lamps emits energy in the form of radiant energy and heat in various amounts. For example, a 400 watt metal halide lamp converts approximately 110 watts to visible energy, 20 watts to UV energy, 70 watts to IR energy, while the remaining 200 watts of energy is converted to heat and dissipated to the surrounding environment via conduction through the lamp base and convection off the glass envelope.
A significant amount of energy is converted to heat by the lamp. In any luminaire design, the heat generated by the lamp can cause problems related to the basic function of the lamp and luminaire. The benefit of effective removal of thermal energy from within the luminaire will be improved luminaire life, smaller package sizes, and in some cases, better lumen output. An additional benefit to removing heat from the luminaire is that the luminaire can then be operated in a higher ambient temperature environment without compromising life or performance.
Additionally, most HID lamps do not re-ignite immediately after a momentary power outage causes them to extinguish. They must be allowed to cool down to an acceptable temperature to allow the arc to be re-ignited. The luminaire and its surroundings can have a significant effect on the length of time it takes an HID lamp to cool down enough to re-ignite. In some applications, an auxiliary lamp (usually quartz) is used to provide backup lighting when momentary power interruptions cause the HID lamp to extinguish. The backup lighting provides minimal acceptable lighting levels until the HID lamp has cooled enough to re-ignite. Occasionally, the auxiliary lamp adds enough heat that the HID lamp never cools down enough to re-ignite. Therefore, an additional benefit of cooling the luminaire is reduced hot re-strike time of an HID lamp.
There are three mechanisms by which thermal energy from the lamp is dissipated: conduction, convection, and radiation. Conduction occurs where physical contact is made between mounting components of the lamp to the lamp housing. Traditional means of providing electrical and mechanical contact between lamp and luminaire provide poor means for conduction to occur between the lamp and external luminaire surfaces. In addition, the location of the lamp and socket are often determined by the desired optical performance of the luminaire. This often necessitates that the socket and lamp be mounted on bosses or other structures that further impede the conductive transfer of heat out of the luminaire envelope, either by creating a longer thermal path, introducing additional thermal interfaces, introducing materials with a lower thermal conductivity, or some combination thereof.
Convection can occur at any surface exposed to air and is limited by the movement of air around the lamp and the difference between the temperature of the lamp surface and the air surrounding it. In many cases, the luminaire may be enclosed, which further exacerbates heat related failures. For example, in luminaires with electronic ballasts and components, the excessive heat can shorten the life of the electronic components causing premature failure of the lighting system.
Radiation is the movement of energy from one point to another via electromagnetic propagation. Much of the radiant energy escapes a luminaire through the optical elements and reflectors. What radiant energy that does not escape is absorbed by the various materials within the luminaire and converted into heat.
Of these three modes of thermal transfer, providing an effective conduction path often allows the greatest amount of controlled heat removal from within a luminaire. This is especially pertinent for luminaires that are enclosed to meet the requirements of the application. Open luminaires can provide good convective energy transfer, but due to limitations of luminaire construction or other application requirements, cannot always provide adequate cooling of the luminaire.
The socket and lamp of many of these luminaire are mounted directly to the lamp housing. The lamp housing contains thermally sensitive electronic components. Even though the luminaire is “open”—a significant amount of heat is transferred to the lamp housing via conduction and convection. By providing an alternative conduction path and dissipation area, a significant reduction in thermal transfer to the lamp housing can be implemented. Good thermal management based on conduction of energy from lamp should be considered.